Male birds react most aggressively to duets from pairs that are in tight tempo, a new study has found. This suggests that better synchrony in a duet signals a larger threat to other magpie larks.

Researchers played recordings of coordinated and uncoordinated duets in 12 magpie lark territories to simulate an "invasion" of other birds.

"An obvious human analogy ... is a human army," said study lead author Michelle Hall, currently at the Max Planck Institute for Ornithology in Germany. She did the research while at the Australian National University in Canberra.

"If you see an army marching and everybody's in time, you sort of wonder about how good they're going to be."

Pairs of magpie larks clock endless hours singing duets in their territories, which they occupy year-round. One bird sings a sound similar to "peewee" while its mate replies "wit."

The degree of synchrony varies widely across pairs. The researchers also found that the duet's coordination, and so its intimidation factor, improves the longer the two sings together.

"Individuals that get their act together" and sing in a extremely highly coordinated way seem to be signaling a commitment to each other in defending territory, said Peter Slater, a professor of natural history at St. Andrews University in Scotland, who was not involved in the study.

Lead author Hall said it's important for their survival and their reproductive success.